- Ovulation is release of the ovum from the ovary due to rupture of the Graafian follicle at the end of the follicular phase.
- Occurs on day 14 of a 28-day cycle; timing is always 14 days before the next menses, since luteal phase duration is constant.
Hormonal Control of Ovulation
- Ovulation occurs due to the midcycle LH surge.
- LH surge is triggered by a positive feedback effect of high estrogen on pituitary LH secretion in the late follicular phase.
- A mild-to-moderate estrogen rise is normally inhibitory (negative feedback) on LH secretion.
- Only a high estrogen level sustained for about 36 hours, reached about 2 days before ovulation, switches this to positive feedback.
- Midcycle FSH surge occurs simultaneously but is not essential for ovulation; it primes follicular cells to express adequate LH receptors.
Mechanism of Follicular Rupture